How the butterfly got its spots?

Back from Toronto, and I blame the jet leg for posting about a paper first published in February.
Anyway, in the paper "Conspicuousness, not eye mimicry, makes "eyespots" effective antipredator signals" published in the journal Behavioral Ecology, researchers from the university of Cambridge claim that butterflies eye spots are not meant to mimic eyes, as most of us were taught in our childhood. Eye spots are just the easiest to produce and most striking pattern butterflies can adorn their wings with. Indeed, in "evo-devo" terms, making circular spots is a rather simple genetic task (excuse me for not using scientific terms). Birds seeing a butterfly with any strong marking allegedly avoid it, in "fear" of it being poisonous.
To test their claims, the researchers pinned dead mealworms to trees in the forests of England, to each they attached a set of wings with different markings. Some worms got eye spots on their artificial wings, some got squares and some got bars. After two days the researchers got back to their "butterflies" and checked how many of them were eaten by the local birds. Their conclusion was that it's not the resemblence of the markings to eyes that caused birds to avoid some of the worms, but rather the size of the markings and their number. You can read more about it here.

Though it makes some sense, I'm a bit skeptical, as much as a non-professional, internet blogger can be. If all that matters is the size and number of spots, why many butterflies have at least spots composed of (at least) two circles of two different colors. Why, many times, is the darker circle the inner circle in the spots, resembling a pupil?